linux-kernel - Re: [PATCH] x86/fpu: Revert earlier patch of Disable AVX when eagerfpu is off

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Message-ID: <20160309124626.GA28660@gmail.com>
Date:	Wed, 9 Mar 2016 13:46:26 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Leonid Shatz <leonid.shatz@...ellosystems.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86/fpu: Revert earlier patch of Disable AVX when
 eagerfpu is off


* Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:

> AVX was mistakenly believed to be dependent on eagerfpu switch.
> This turns out to be false. The earlier patch should be reverted.
> 
> Original patch:
>     http://git.kernel.org/tip/394db20ca240741a08d472173db13d6f6a6e5a28

So the original patch had a whole host of explanations of why that change is 
correct. This revert should explain where that argumentation was wrong.

Also note that eagerfpu=off is on the way out, on v4.6 we'll switch all CPUs to 
eagerfpu:

  58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs

and in the not so distant future, if everything goes fine with the eager mode, I'd 
like to drop the lazy FPU context switching code altogether - which will simplify 
a lot of code. At that point the 'eagerfpu' boot option will go away as well.

Thanks,

	Ingo